1. Field of Invention
The invention relates to a liquid crystal device (LCD) and, in particular, to a control circuit for a common line.
2. Related Art
Liquid crystal devices (LCD's) have the advantages of high image quality, small size, light weight, low driving voltage, low power consumption, and wide applications. Therefore, they are often used in mid- and small-size portable televisions, mobile phones, video cameras, laptop computers, desktop displays, and projective televisions. Gradually, they replace the cathode ray tube (CRT) monitors and become the mainstream of display devices. Because of its high display quality and low power consumption, the thin film transistor (TFT) LCD has occupied the majority of the market.
The primary components of an LCD are liquid crystal pixels disposed in an array sandwiched and enclosed between two substrates. One of the substrates provides a pixel electrode, and the other provides a common electrode. The TFT LCD uses a single TFT to impose a voltage on the corresponding pixel electrode. The crystal axis direction is then determined by the potential difference between the pixel electrode and the common electrode, thereby controlling whether the local crystal is transparent or opaque.
Generally speaking, the TFT's for the crystal liquid pixels in the same row are controlled by one scan line. The potential of the associated common electrode is controlled by a common line. With reference to FIG. 1A, the scan lines 102 (Gn˜Gn+3) are used to control the TFT's of liquid crystal pixels in different rows. However, their common lines 104 are connected together. That is, liquid crystal pixels of different rows have the same common electrode potential. When the common electrode potential of one of the rows is changed, those of other rows will change accordingly.
With the advance in the technology of LCD's, there are some new methods, such as common modulating or other special techniques, for improving the operation of liquid crystal pixels. From FIG. 1B, we see that the conventional method only uses the potential 112a on the pixel electrode to change the voltage imposed on the liquid crystal pixels, while the potential 114a on the common electrode is maintained at a constant. Therefore, if one wants the potential difference a imposed on a liquid crystal pixel of two adjacent frame times to be equal, the change in the potential 112a has to be two times a.
With reference to FIG. 1C, the common modulating method simultaneously uses both the potential 112b on the pixel electrode and the potential 114b on the common electrode to change the voltage imposed on the liquid crystal pixels. Since the common electrode potential 114b is also employed to change the voltage on a pixel, the change in the pixel electrode potential 112b only needs to be half that of the conventional method in order for the potential difference a imposed on the liquid crystal in two frame times to be the same.
From the above description, we see that if one uses the common modulating method to operate the liquid crystal pixels, the potential on the common line has to be changed periodically with the frame time. The potential of the common line in other special applications often needs to be changed too. Owing to the existing driving means in LCD's, the common lines of different rows have to be connected together (as shown in FIG. 1A). Therefore, if the common electrode potential of a particular row is to be varied, the common electrode potential on the whole panel has to be changed accordingly at the same time. If the common electrode potential has to be changed frequently because they are connected together, there are the following drawbacks:                1. High switch frequencies will result in large power consumption;        2. High impedance of the common electrode may easily produce horizontal cross-talk; and        3. Asynchronous switching between the scan line and the common line will generate perturbations on the potential difference of the liquid crystal pixels, seriously affecting the LCD image quality.        